It is obviously desirable for coatings that are to be applied to ovens, boilers, heat exchangers, automotive parts, cooking elements, cooking utensils and the like to exhibit high temperature resistance. Most organic coatings are unsuitable for such applications as they tend to be rapidly consumed when exposed to air at temperatures greater than 550° C. This consequence lead to the development of coatings and paints that incorporated polysiloxane resins, as described in U.S. Pat. No. 5,905,104 (Eklund et al.). In the examples a mixture of siloxane resins is mentioned, viz. a mixture of Dow Corning 1-0543 and Dow Corning Z-6018. These resins have the following properties:                Dow Corning 1-0543 (now DC 220) Tg=49° C., viscosity at 140° C. of 9.8 poise;        Dow Corning Z-6018 Tg=48° C., viscosity at 140° C. of 14.1 poise.        
Despite showing improved temperature resistance, coatings containing polysiloxane resins were still found to exhibit deleterious effects at the high temperatures. When polysiloxane powder coated materials are exposed to temperatures greater than 550° C., the coatings suffered loss of their constituent organic components through oxidation; the polysiloxane resin consequently shrinks rapidly which builds up stresses within the coatings. Such stresses are relieved by cracking causing the coating to peel or flake from the material.
WO2004/076572 (Dupont de Nemours and Company) purports to resolve this problem by including within the polysiloxane resin at least one matrix material, preferably low melting inorganic glass, that softens and exhibit some flow in the temperature range in which the polysiloxane resin undergoes shrinkage and embrittlement.
European Patent No. 0950695 B1 (Morton) proposes an alternative solution in which the powder coating composition consists of a single silicone resin combined with titania and a filler of mica platelets and/or calcium metasilicate particles. The single silicone resin is characterized by having siloxane functionality (Si—O—H) and only minor amounts of organic moieties. It is preferred in this citation that the single polysiloxane has a degree of substitution of less than 1.5 and an —OH content of between 2.0 and 7.5 wt. % based on the weight of said polysiloxane. The limitation of the —OH content reduces the evolution of water when the polysiloxane self-cures at temperatures between 150° and 260° C. and thus reduces the formation of defects, such as pinholes, in the coating that are caused by said water escaping. However, it is noted this powder coating composition may only be applied to substrates at a dry film thickness in the range from 1.8 to 2.2 mils (45 to 55 μm).
WO 2009/003937 (Akzo Nobel Coatings International B.V.) discloses a powder coating composition which comprises a resin component and a filler, wherein the resin component comprises a first silicone resin and a second silicone resin having glass transition temperatures (Tg) that are different by at least 5° C. and/or having melt viscosities, as measured at 140° C., that are different by at least 5, preferably 10 poise. With respect to the filler component of the composition, it is preferred that the filler is a heat resistant material with one dimension at least four times larger than the other, said filler being present in an amount between 5 and 95 wt. % based on the weight of the resin component. It was found that these compositions are not able to withstand prolonged exposure to high temperatures (˜550° C.) when applied to substrates with a high surface roughness, i.e. substrates that have a surface that is profiled or uneven.
When powder coatings are applied to automotive bodies in order to protect and finish the engineered product, the substrates tend to be relatively thin and to have smooth surfaces. However, in the application of coatings to materials that are required to show high temperature resistance, it is more common for the substrate surfaces to be profiled or uneven: to provide adequate corrosion protection and an (aesthetic) finish to blast cleaned steel, for example, the substrate must be coated at a sufficient dry film thickness to compensate for surface unevenness. Blasting substrates with angular grit, rounded shot, abrasive loaded sponges or high pressure water jets can typically yield profiled surfaces that can exhibit “valley to peak” distances of between 10 and 80 μm (wherein said profiles may be defined by ISO 8503).
For such uneven substrates, there is found to be a practical upper limit to the dry film thickness (DFT) of the powder coating, beyond which the coating will crack and peel from the substrate. Obviously, the lower that limit, the lower the capacity of a given powder coating to compensate for enhanced blast profiles of a substrate surface.
There consequently exists a need in the art to provide a powder coating composition that shows high temperature resistance but which also may be applied to profiled substrate surfaces to provide temperature resistance and preferably corrosion resistance to said surfaces.